An electric storage device having a high voltage and a high energy density has been required as a power source for driving an electronic device. Especially lithium ion batteries and lithium ion capacitors are expected as electric storage devices having a high voltage and a high energy density.
An electrode used in these electric storage devices is generally manufactured by applying a mixture of active material particles and polymer particles which function as an electrode binder to the surface of a current collector and drying it. Characteristic properties required for the polymer particles used in an electrode include bonding ability between active material particles, integrity with the active material particles and the current collector, abrasion resistance in the step of winding the electrode and powder drop resistance that fine particles of the active material do not drop from a composition layer for electrodes (to be simply referred to as “active material layer” hereinafter) by cutting after the above step. When the polymer particles satisfy the above requirements, the method of folding the obtained electrode and the design of the structure of an electric storage device such as the setting of a winding radius have a high degree of freedom, thereby making it possible to reduce the size of the device. It has been empirically found that the above bonding ability between active material particles, integrity with the active material particles and the current collector and powder drop resistance are almost proportional to one another. Therefore, these properties may be collectively represented by the term “adhesion” hereinafter in this text.
It is advantageous that a fluorine-containing organic polymer having excellent ion conductivity and oxidation resistance, such as polyvinylidene difluoride (PVDF), should be used as an electrode binder. However, since an organic polymer containing a fluorine atom is generally interior in adhesion, the obtained electrode has problems with mechanical strength and durability. Then, technologies for improving adhesion while retaining the ion conductivity and oxidation resistance of an organic polymer have been investigated and proposed.
For example, Patent Document 1 (JP-A 2011-3529) proposes a technology for obtaining the lithium ion conductivity and oxidation resistance as well as adhesion of a binder for negative electrodes by using both PVDF and a rubber-based polymer. Patent Document 2 (JP-A 2010-55847) proposes a technology for improving adhesion through the step of removing a solvent at a low temperature after PVDF is dissolved in a specific organic solvent and the resulting solution is applied to the surface of a current collector. Further, Patent Document 3 (JP-A 2002-42819) proposes a technology for improving adhesion by using an electrode binder having a side chain with a fluorine atom in the main chain composed of a vinylidene fluoride copolymer.